Cristina Figueres, a former top United Nations official who was in charge of negotiations on the Paris agreement, introduced Brown at the Global Citizen concert as a "climate optimist".

The key risk for defenders of the global pact is that other countries could now follow Trump's lead.

In a summit with its fair share of odd and awkward moments, it was Ivanka's appearance at one discussion alongside Xi, Merkel and Erdogan that caused the biggest stir.

It has also been about efforts by most of the rest of the world to cajole the US president into softening his stances on global trade and the climate, with Merkel in a secondary role, trying to come up with compromises.

On trade, difficult talks yielded the preservation of the G20's condemnation of protectionism, a statement that has been a hallmark of the group's efforts to combat the global financial crisis and the after-effects of the Great Recession.

"I think [Brown's] message was really important". "The leaders of the other G20 members state that the Paris agreement is irreversible" and "we reaffirm our strong commitment to the Paris agreement".

The summit set for 2018 in San Francisco will mark the first time a state is hosting a climate gathering aimed at upholding the goals of the Paris deal.

In most other respects, though, the summit meeting had to be a bitter disappointment for the chancellor. But Tillerson, who was present at the meeting that ran two hours and 15 minutes, also said the two alpha-male leaders "connected very quickly" with "very clear positive chemistry".

Climate change policy proved a sticking point, with the United States pressing for inclusion of wording about which other countries had reservations.

Her standing has also suffered as Germans have been shocked by violent protests by a small bloc of anarchists who saw the G20 as a perfect platform for their rejection of capitalism and order.

Trump in June announced that the US will withdraw from the Paris climate accord, saying the deal agreed by more than 190 nations unfairly benefited countries like India and China.

Jerry Brown plans to convene a climate conference next year, his latest action to position the state as a leader in battling global warming as the White House recedes. Erdogan's threat brought further disarray to a summit that was marred by bilateral quarrels and strife over climate protection and trade.

He added that Prime Minister had made a "huge commitment" to implement the Paris agreement.

For Merkel, the summit is an opportunity to show off her diplomatic skills ahead of a federal election in September, when she is seeking a fourth term in office.

The targets for carbon emissions reductions for each country and provisions to help poorer countries adapt to an already-changed planet were negotiated for two decades before the December 2015 signing.

Historian Anne Applebaum took to Twitter to denounce what she described "an unelected, unqualified, unprepared New York socialite" being seen as "the best person to represent American national interests".

"We would like to get it agreed by 2019".

His budget, released in April but largely ignored on Capitol Hill, would include deep cuts to the US Agency for International Development, a major conduit for foreign assistance.